This invention relates to the type of laser beam deflector that utilizes the diffraction of light by acoustic waves. In such acousto-optic deflectors an electrical input signal is fed to a transducer that is bonded to a deflector medium. The transducer can be excited and launches acoustic waves into the medium. Under appropriate conditions, an incident laser beam may be deflected by the acoustic wave to many diffraction orders. The angle of deflection can be scanned by changing the frequency of the acoustic waves.
In a laser deflector, one of the most important performance parameters is the resolution, or maximum number of angular positions, which is defined as the ratio of total deflection angle to the optical divergence of the laser beam. For conventional acousto-optic deflectors where Bragg diffraction in the first order is always use, the resolution N is equal to the product of total scanning frequency bandwidth .DELTA.f and the acoustic transit time .tau. across the optical aperture, i.e., N = .DELTA.f.tau.. Bandwidth is limited by acoustic attenuation at higher frequency, and the acoustic transit time is usually limited by the access time requirement or spatial constraint. One method to improve the resolution is the use of optically cascaded deflectors which was reported by Watson and Adler in a paper published in 1969 IEEE Conference on Laser Engineering and Applications. For efficient operation of the deflectors, special beam steering had to be used and the cells had to be critically aligned. The complexity of this approach makes it less attractive. In a paper entitled "Continuous Deflection of Laser Beams," which appears in Applied Physics Letters, Vol. 10, of January, 1967, pages 48-51, Lean et al. reported the deflection of laser beam by acoustic waves in birefringent LiNbO.sub.3 crystal. At a specific acoustic frequency as determined by the crystal birefringence and optical wavelength, the momentum matching condition for the acousto-optic diffraction is approximately satisfied over a broad range of acoustic frequencies. This has been referred to as the 90.degree. phase matching birefringent diffraction since in this case the diffracted light wave vector is perpendicular to the acoustic wave vector. Based on the same concept. Warner et al. described the operation of a broadband acousto-optic deflector using the circular birefringence in paratellurite; this appeared in the Journal of Applied Physics, Vol. 43, November of 1972, pages 4489-4495. The authors also reported that due to rediffraction the peak diffraction efficiency to first order is limited to below 50%.